1. Field of the Invention
The present invention pertains to a method for braking an aircraft during landing rollout. With greater particularity, the present invention pertains to a method of applying braking force so that the aircraft will not experience hydraulic fluid pressure overshoot or deceleration overshoot, regardless of the model and internal volume of the braking system installed on the aircraft. With greatest particularity, the present invention pertains to such a method that uses an analog pressure sensor and closed loop feedback control of hydraulic fluid pressure to maintain brake stack closure condition and select the appropriate control law for incrementing the autobrake control command.
2. Description of the Related Art
Aircraft brakes of the type used in large commercial airliners typically use a series of rotating members that turn with the aircraft wheels (rotors) interleaved with stationary members (stators) that do not turn, not unlike a multiple disk clutch. Pressure forcing the rotors and stators together producing a compressed stack of rotors and stators develops frictional forces between the rotors and stators that resists rotation of the rotors and slows the aircraft. The number of rotors and stators is chosen to achieve the frictional surface area required to develop the torque necessary to slow down or stop the aircraft smoothly and within a reasonable distance. Retarding forces are generated only after the rotors and stators are compressed to the point they are in firm contact with one another, the point known as "brake stack closure."
To prevent the brakes dragging, hydraulic force member 22 controlling compression of the rotors and stators must be held in a retracted position short of brake stack closure when braking force is not desired. Because large commercial airliners are heavy, braking systems for those airliners frequently require large force inputs to achieve sufficient retarding force to slow or stop the forward motion.
Autobraking systems have been devised to apply brake force automatically on landing according to a predefined schedule, and in response to aircraft deceleration from the combined effects of brakes, engine thrust reversers and braking flaps. Also, anti-skid systems have been devised and implemented to prevent locked wheels or skids when landing surfaces are slippery.
If an aircraft autobraking system applies too much or not enough hydraulic fluid pressure than the designers intended at any point in the landing sequence, the autobraking system is said to have a pressure overshoot or undershoot. Similarly, if aircraft deceleration is greater than or less than the designers intended at any point in the landing sequence the autobraking system is said to have a deceleration overshoot or undershoot. These conditions can interfere with a smooth landing rollout as well as cause discomfort and apprehension to passengers and crew.
In the past, aircraft autobraking systems have typically used rotors and stators manufactured from steel, and have used open loop control systems to control autobraking function to achieve brake stack closure. These autobraking systems normally do not experience the problems of deceleration overshoot where the brakes generate more stopping torque than desired and so open loop control systems were adequate to achieve the initial stack closure. However, when modern commercial aircraft began using lighter weight carbon fiber rotors and stators, overshoot and undershoot problems became significant enough to demand solutions.
Usually, pressure overshoot is caused by the autobraking control system being unaware of the exact relation of the rotors and stators to brake stack closure, and applying a scheduled though inappropriately large amount of pressure to the hydraulic system as the brakes are applied.
Previous autobrake systems use carefully tailored open-loop valve command profiles (brake fill spike) to set initial brake pressure at the point where the brake friction elements are just beginning to produce torque (brake stack closure). This is achieved by metering a preset volume of brake fluid into the brakes from the autobrake pressure control valve.
Variability in the volume of the brakes and in the response of the autobrake pressure control valve often results in an initial brake pressure which is above or below the desired value. When the initial pressure is too high (brake overfill), an objectionable deceleration overshoot results from the sudden onset of brake torque. When the initial pressure is too low (brake underfill), subsequent closed-loop deceleration control commands an increasing pressure rise rate to the brakes because the lack of torque output results in an increasingly large error between commanded deceleration and actual deceleration. This results in brake pressure overshoot with accompanying deceleration overshoot at the moment of stack closure.
The problem of obtaining correct brake stack closure using an automated braking system is compounded on some modern airplanes because the aircraft manufacturer offers the airplanes with optional braking systems from different brake system manufacturers. The volume of hydraulic fluid required to fill the brake system sufficient to move the rotors and stators to the brake stack closure point may be much larger for one style of brake than for another. Therefore, if the control system doesn't take brake system design differences between manufacturers into account, the use of a preset hydraulic fluid volume that works well to achieve brake stack closure for one brake style may result in deceleration overshoots with the other. Aircraft manufacturers regard it as advantageous to have a single autobraking control system rather than several tailored to the design of each different brake manufacturer's braking system.
A related problem occurs when the autobrake system commands the brake to remain at brake stack closure pressure when the required deceleration is met or exceeded by other retarding devices such as thrust reversers or wing spoilers. The existing technology for manufacturing autobrake pressure control valves does not permit the precision necessary to closely control output pressure during open-loop command. As a result, the brake pressure may be set too high causing objectionable brake drag, or too low which results in deceleration overshoot when the autobrake control system returns to closed-loop deceleration control.
Prior attempts to obtain brake stack closure relied on the selection of an open loop brake fill spike that would just fill the volume of an average set of brakes and a shape of the fill spike command (current versus time) that would minimize the effect of variations between autobrake valves. Variations in the actual volume of the brake and in the performance of the autobrake valve still caused some airplanes to experience objectionable deceleration behavior. A second approach used a different fill spike for each style of brake. This requires configuration control to differentiate between autobrake controllers with different software versions so that the controller can be matched to brake style installed.
Prior attempts to hold brake pressure at the stack closure level, while deceleration is being provided by thrust reversers, have relied on setting the brake pressure above the brake stack closure pressure of a high brake stack closure brake by a specified amount determined by analysis to ensure that all brakes are closed. This reduces the probability of deceleration overshoot by preventing brake underfill. A side effect of this technique is objectionable brake drag. Dragging brakes generate excessive deceleration, increase brake wear, and require longer waiting time to allow the brake temperature to drop to acceptable levels before the next take-off. The latter adversely affects the airlines' ability for quick turnaround for "short hop" flights. These problems and others have been solved by the present invention.